The effect of ammonia reduction on the spinel electrode materials, LiMn2O4 and Li(LiMn)O4

Treating the lithium manganese oxide spinels, LiMn2O4 and Li(Li13Mn53)O4 with ammonia at elevated temperatures causes oxygen loss. The reaction between these materials and ammonia as a function of temperature has been studied using X-ray diffraction and Thermal Gravimetric Analysis (TGA). Electrochemical studies of the subsequent deintercalation of Li from these materials have been made using Li/non-aqueous electrolyte/spinel cells. Very little oxygen can be removed from LiMn2O4 (synthesized at 750°C) by NH3 reduction before disproportionation to Li2Mn2O4 and Mn3O4 occurs. By contrast, treatment of Li(Li13Mn53)O4 and LiMn2O4 (both synthesized at 400°C) with ammonia at temperatures near 200°C removes oxygen without inducing a phase change. For Li(Li13Mn53)O4, the product of the reduction is best characterized as Li(Li13Mn53)O4−δ with δ as large as 0.25. Unlike LiMn2O4 (prepared at 750°C), where half the Mn is in the 3+ oxidation state and almost all the Li can be removed electrochemically, very little Li can be removed from Li(Li13Mn53)O4 (all Mn4+) because it is apparently difficult to oxidize the Mn atoms in it beyond the 4+ state. When δ increases, the average oxidation state of Mn decreases and then substantial Li can be deintercalated. Similarly, reducing LiMn2O4 (prepared at 400°C) with ammonia increases the amount of lithium which can be extracted from the material. These results demonstrate the striking effect of the Mn:O ratio, or equivalently the Mn oxidation state, on the electrochemical behavior of Li/spinel cells.